Known for protecting pumps against dry running is to measure the electrical conductivity between the pump housing of the pump and the liquid to be pumped. Without liquid, there exists no electrically conductive contact to the pump housing, and there is an extreme rise in resistance. The disadvantage to such measuring systems is that the electrical contact greatly depends on the electrical conductivity of the liquid, and that it most often takes a certain period of time to detect dry running.
Known from EP 1 510 698 A2 is an oscillating fork arranged in a tube flange on the pump for detecting the dry running of a pump. The oscillating fork is made to oscillate at regular intervals. If the oscillating fork is only in contact with air, the oscillation has a longer decay time than when the oscillating fork is in contact with water given the low attenuation. The disadvantage to this design is that an additional tube flange must be secured to the pump to provide enough space for the oscillating fork. In addition, excitation here takes place at a low frequency. As a result, it may be necessary to determine the decay time with several excitations, so as to thereby be able to prevent the excitation from exerting an influence on dry running detection itself.